A. Field of the Invention
The present invention is related to rotary kilns, used in the cement or similar industries having a calciner and a tertiary air duct and more particularly to a method and apparatus which uses the hot air from the calciner tertiary air feeding duct for burning waste material and recovering the energy from the burned material.
B. Background of the Invention
The necessity to reduce total fuel costs through the substitution of waste-derived fuels instead the normal fossil fuels, primarily coal or pet coque, has been the main driving force behind the desirability of cheap alternative fuels to fire industrial kilns such as rotary kilns for producing cement.
In previous art, alternative solid fuels are normally fed at the kiln entrance. The amount and characteristics of alternative solid fuels which can be burned in the industrial kiln such as discarded tires and industrial waste depends on the amount of available oxygen at the kiln entrance.
In kilns having a calciner, only about the 50% of the total amount of oxygen provided to the industrial kiln is feed to the kiln and the other 50% is feed to the calciner, which considerably restricts the amount of waste material that can be burned as alternative solid fuel.
When the waste material is burned at the kiln entrance, the oxygen in such place, which is between 3 to 5%, is consumed, producing a reducing atmosphere which favours the formation of solid scales on the kiln and calciner wall thus reducing the overall efficiency of the industrial kiln.
There have been developed many external combustion chambers or gasifiers which try to avoid the above referred problem, but since the acquisition, implementation and overall operational cost of such apparatuses are high the use of an external device is not completely desirable.
An attempt to use alternative solid fuels without reducing the amount of oxygen at the kiln and without using an external combustor or gasifier, has been made by injecting and burning the waste material directly into the tertiary air duct which is described in the U.S. Pat. No. 6,470,812.
U.S. Pat. No. 6,470,812 describes a method and apparatus to recover energy from waste preferably solids whether in bulk or crushed, such as vehicle tires, bags, bales, bulk material that may be contained in tanks, barrels, etc. by means of combustion of such waste in industrial furnaces, particularly of the rotary type, that has an external calciner, feeding such waste into the tertiary air duct of the calciner and burning the waste inside the duct itself.
The method further discloses removing from the tertiary air duct any solid residues remaining therein from the decomposed waste and the apparatus comprises a feed mechanism for external charging of the waste directly into said tertiary air duct and a separating mechanism for removing from the tertiary air duct any solid residues remaining therein from decomposed waste.
As stated by the U.S. Pat. No. 6,470,812, the feed to the duct preferably should be about 1 to 5 meters from its discharge end. Since feeding waste material to the exhaust gas outlet results in build up of encrustations, the feed to the tertiary air duct should be sufficiently remote from the duct discharge into the exhaust gas to allow adequate residence time of the waste-derived products in the duct to prevent such build up from occurring.
However it may be possible that some of the waste material end up burning near the exhaust gas, specially when feeding big pieces of waste material such as whole tires, thus producing encrustations and requiring periodic plant shut down for its removal. Therefore it is very important that the waste material be feed in small batches of material in order to guarantee that such small batches burn completely before arriving to the exhaust gas outlet. It is clear that the use of batches of whole tires by the above referred system is very restricted.
Furthermore, since the above referred system teaches to remove detritus such as the steel wires from car tires, if amount of wire is excessive, it may become entangled and the mechanism for removing any solid residues may have problems to handle the entangled wire.
Therefore it would be highly desirable to have a system which can be able to burn high amounts of waste material including whole tires, without producing a reducing atmosphere at any point of the industrial kiln nor the formation of solid scales on the kiln or calciner wall.
In view of the above referred need, applicant developed a method and apparatus which uses a less than 10% of the hot tertiary air in order to burn the waste material, specially whole tires and which further incorporates to the cement clinker any material that may be compatible in cement such as steel wires from tires.
Applicant's apparatus comprise: one or more combustion chambers formed into the calciner tertiary air feeding duct, each having releasable retaining means therein for retaining the waste material inside the combustion chamber until a complete combustion is achieved since at this point there is an excess of oxygen of more than a 300% and the air temperature is between 650° to 950° C., said releasable retaining means allowing the flow of combustion gas and air trough itself; and one or more feeding means each connected to a combustion chamber for feeding waste material to the combustion chamber further avoiding the loss of heat or the inlet of cold air from the exterior into the hot tertiary air stream trough the feeding means.
Similarly applicant's method comprises: burning waste material inside a combustion chamber formed into the calciner tertiary air feeding duct using a fraction of tertiary air and retaining the material therein for a predetermined amount of time for allowing a complete decomposition of the combustible portion of the waste and a complete combustion of the waste achieved since at this point there is an excess of oxygen of more than a 300% and the air temperature is between 650° to 950° C.
The main advantage of applicant's apparatus and method comprises the fact that the whole combustion process is carried out inside the calciner tertiary air feeding duct, which completely avoids the possibility of burning the waste material at the kiln entrance or in any other place of the kiln.
Since the non combustible material is directly feed by gravity to the combustion chamber, there is no need to have means for retiring any remaining material, and remaining material such as steel from tires can be fully used and incorporated in the cement clinker.
Furthermore, the modification of current rotary kilns in order to be adapted to use the apparatus of the present invention is not complicated, and since the system is not comprised by complicated equipment, the acquisition, implementation and operational costs are very low.
With the apparatus of the present invention, it is achieved a decrease of pet coque consume of from 10 to 30% per combustion chamber.